1. Field of the Disclosure
This invention relates to integrated inductor structures, especially to highly compact integrated inductor structures.
2. Description of Related Art
Due to manufacturing restrictions, on-chip inductors are often in the design of planner structures. FIG. 1 illustrates a prior-art planner spiral inductor. The planner spiral inductor 100 includes a spiral first metal trace 110 (light color), and a second metal trace 120 (dark color). The first metal trace 110 and the second metal trace 120 are deployed in different layers of a semiconductor structure. In FIG. 1 the first metal trace 110 is above the second metal trace 120, but it is also possible that the first metal trace 110 is under the second metal trace 120. The first metal trace 110 and the second metal trace 120 are connected through a connecting structure 130. The first metal trace 110 includes a coil of 3 turns. When there is a need to increase the inductance of the planner spiral inductor 100, the number of turns of the coil of the first metal trace 110 must be increased. The increase in the number of turns not only causes an increase in the area of the planner spiral inductor 100, but also causes an increase in the parasitic series resistance and the parasitic capacitance of the planner spiral inductor 100, which in turn causes the self-resonant frequency and the quality factor Q of the planner spiral inductor 100 to decrease. In addition, metal loss and substrate loss are also key factors to the quality factor Q. The metal loss is caused by the resistance of the metal itself. The substrate loss arises from two reasons. One is that when the inductor operates, a time-varying electric displacement happens between a metal coil of the inductor and a substrate, which cause a displacement current between the metal coil and the substrate. When the displacement current penetrates into the low resistance substrate, energy losses occur. The displacement current is related to an area of the coil, the larger the area, the greater the displacement current. The other reason is that a time-varying electromagnetic field of the inductor penetrates a dielectric layer, which causes a magnetically induced eddy current on the substrate. Energy losses occur due to the opposite directions of the induced current and the current of the inductor.
When an inductor operates in low frequencies, the current distributes evenly in the metal coil and the metal loss at the time is from the series resistance in the metal coil. When the inductor operates in high frequencies, greater magnetic fields are induced at inner turns of the metal coil than at outer turns. Intense magnetic fields induce eddy currents at inner turns of the metal coil. The eddy currents causes uneven distribution of currents, most currents being pushed to the surface of the metal coil, which is known as a skin effect. Under the skin effect, the cross section of the metal through which the current flows becomes smaller, and hence the current experiences a larger resistance, which results in a decreased quality factor Q. FIG. 2 shows another prior-art planner spiral inductor. This tapered spiral inductor alleviates the skin effect since the inner turns of the metal coil of the inductor suffer the most severe skin effect. Further, the tapered spiral structure reduces the area of the inductor and decreases the parasitic capacitance so the quality factor Q and the self-resonant frequency of the inductor can be improved. Because of its asymmetric structure, a position of the inductor's center tap is hard to decide. Moreover, the positions of an inductive center, a capacitive center, and a resistive center of this spiral inductor are different from one another, which makes this spiral inductor improper for passive components in a differential circuit.
To address the problem, a symmetric spiral inductor is introduced. FIG. 3 shows a structure of a prior-art symmetric spiral inductor. The symmetric spiral inductor 300 includes a plurality of metal traces 310 (310a˜310d), a plurality of connecting traces 320 (320a˜320c), and a plurality of connecting structures 330. A connecting trace 320 is also referred to as a bridge. All the metal traces 310a˜310d are deployed on the same layer of a semiconductor structure (light color), and all the connecting traces 320a˜320c are made of metal and are deployed on a different layer (dark color) from the metal traces 310a˜310d. As an example, the connecting traces 320a˜320c are deployed under the metal traces 310a˜310d in FIG. 3.
The connecting traces 320 are to connect different metal traces 310. For example, the connecting trace 320a connects the metal trace 310a and the metal trace 310b, the connecting trace 320b connects the metal trace 310b and the metal trace 310c, and the connecting trace 320c connects the metal trace 310c and the metal trace 310d. The two ends of a connecting trace 320 connect the metal traces 310 through the connecting structures 330. The connecting structure 330 can be a via structure of a semiconductor manufacturing process that connects components at different layers. It is easy to find the position of the center tap of the symmetric spiral inductor 300 for its symmetric structure; however the connecting traces 320 are frequently used. When an inductor with large inductance is required, the turns of the symmetric spiral inductor 300 must be increased, which in turn increases the number of the connecting traces 320. If the sheet resistance of the connecting trace 320 is larger than that of the metal trace 310, then the quality factor Q of the symmetric spiral inductor 300 is dominated by the resistance of the connecting traces 320 and the parasitic resistance of the connecting structures 330.
FIGS. 4A and 4B illustrate a structure of another prior-art symmetric spiral inductor and its partial enlargement. The symmetric spiral inductor 400 includes a plurality of metal traces 410 (410a˜410d), a plurality of connecting traces 420 (420a˜420c), and a plurality of connecting structures 430. Similar to FIG. 3, the metal traces 410 are connected by the connecting traces 420. The two ends of a connecting trace 420 connect the metal traces 410 through a connecting structure 430, respectively. In contrast to the structure in FIG. 3 which is rectangular, the structure of the inductor in FIG. 4 is octagonal and therefore has better inductive effects. FIG. 4B shows a partial enlargement (corresponding to the area enclosed by a dotted line in FIG. 4A) of the symmetric spiral inductor 400. Generally a cross section of the connecting structure 430 is designed to be a rectangle. Due to restrictions of the IC design rule, the width of the connecting structure 430 must be greater than D, and a distance to the edge of the metal trace 410b must be greater than h. In other words, if the width of the connecting structure 430 is designed to be D, the width W of a connecting area 440 (depicted by stripped lines) of the metal trace 410b through which the connecting structure 430 connects the metal trace 410b must be greater than D+2h. For example, if D is 3 μm and h is 0.5 μm, then W must be greater than 4 μm. Likewise, the connecting trace 420b has a connecting area on the corresponding position through which the connecting structure 430 connects the connecting trace 420b, and the width of this connecting area must be greater than W as well. In general, the width of the connecting trace 420b is designed to be W. Because the widths of the metal traces 410 and the connecting traces 420 of the symmetric spiral inductor 400 are designed to be W, the flexibility of designing the entire area of the symmetric spiral inductor 400 is restricted; in particular, when the turns of the symmetric spiral inductor 400 are increased to enhance the inductance, the increase in the entire area of the inductor causes an increase in the parasitic capacitance and a decrease in the self-resonant frequency.